Development of myelin, a fatty sheath that insulates nerve fibers, is critical for brain function. Myelination during infancy has been studied in postmortem histology, but such data cannot evaluate the developmental trajectory of the white matter bundles of the brain. To address this gap in knowledge, we (i) obtained longitudinal diffusion MRI measures and quantitative MRI measures of T1, which is sensitive to myelin, from newborns to 6-months-old infants, and (ii) developed an automated fiber quantification method that identifies bundles from dMRI and quantifies their T1 development in infants. Here we show that both along the length of each bundle and across bundles, T1 decreases from newborns to 6 months-old’s and the rate of T1 decrease is inversely correlated with T1 at birth. As lower T1 indicates more myelin, these data suggest that in early infancy white matter bundles myelinate at different rates such that less mature bundles at birth develop faster to catch-up with the other bundles. We hypothesize that this development reflects experience-dependent myelination, which may promote efficient and coordinated neural communication. These findings open new avenues to measure typical and atypical white matter development in early infancy, which has important implications for early identification of neurodevelopmental disorders.
Factors associated with catch‐up growth in early infancy in rural Pakistan: A longitudinal analysis of the women's work and nutrition study